The present invention relates broadly to a steerable radar apparatus, and in particular to a survivable ground base steerable array antenna elements.
It has become a matter of prime importance in defense to provide hardened sites for our missiles. The philosophy behind the use of these weapons is one of retaliation. It is obvious therefore that these weapons must be protected as far as possible against the first strike situation. However, the protection of the weapons alone would be of minimal value if command communications and advance warning radar systems were lost during an initial attack. It is, therefore, with great need that hardened sites be provided in order to protect radar warning system in order to maintain an effective tactical warning system in a post attack situation.
In a conventional phased array which coherently combines signals from all array elements, spacing radiating elements apart by more than a fraction of a wavelength produces numerous grating lobes. This precludes any practical use of widely spaced elements. The distributed array of the present invention uses noncoherent addition of radar return signals that are received at each subarray from signals that are transmitted by all of the subarrays. In the noncoherent array approach, the 60 subarrays can be widely spaced yet not produce these objectionable grating lobes.
A novel form of time-delay steering is used to position the array's beam in space. Each subarray transmits a single pulse at a different frequency, and the time of transmission is adjusted so that all 60 pulses arrive at some selected azimuth and range in time coincidence. If there is a target at this range and azimuth, the 60 time coincident pulses are reflected back to the array of 60 subarrays. Each subarray has a receiver that contains 60 separate channels tuned to the 60 frequencies that are transmitted by all the subarrays. The output of these channels are noncoherently added producing a detectable signal. The signal-to-noise ratio in each of the 60 individual channels is too small to insure reliable detection, but noncoherent addition of all channels provides sufficient signal-to-noise ratio so that the probability of detection of the target is over 90% at each subarray. Target reports are generated at each subarray and connected to the Tactical Warning system through a suitable communications architecture.